Electrooptical transmission system



REYNLS ELECTROOPICAL TRANSMISSION SYSTEM Filed Fem 18 132? 2 Sheetsmmsi REYNQLQ ELECTROOPTICAL TRANSMISSION SYSTEM N Filed Feb." la, 192? 2 sheets-sheet 2 Patented Aug. 19, 1930 UNITED STATES PATENT oFFics FREDERICK W. REYNOLDS, OF GRANTWOOD, NEW JERSEY, ASSIGNOB TO AMERICAN TELEPHONE AND TELEGBAPH COMPANY, A CORPORATION OF NEW YORK ELECTROOPTICAL TRANSMISSION SYSTEM; i

Application led February 18, 1927. Serial No. 169,322.

This invention Vrelates to electro-optical transmission and more particularly to the electrical transmission of ictures.

An object of the invention is the simplification of the terminal equipment by the em` ployment of certain elements for either the transmission or the rece tion of ictures. This invention-is particu arly applicable to the-transmission of ictures in either direction over telephone circuits.

In an embodiment of the invention selected for illustration a string or ribbon electromechanical light valve such as disclosed in Patent No. 1,667,805, May 1, 1928, of Herbert E.

' Ives, is positioned between a source of light and a holder for the transmitting or receiving lilm, depending upon whether transmission or reception is taking place. For picture transmission the light valve ribbon is vi- 2o brated b alternating current from a vacuum tube oscillator or other suitable source at a substantially constant frequency thereby generating a ulsating light beam which is here-4 inafter ca led a carrier light beam. This carrier light beam is modulated by the picture at the transmitter and is subsequently directed onto a photoelectric cell which in turn produces pulsating photoelectric currents representative of the light effects of` ao the picture. By means of a transformer, these currents set up alternating current which constitutes the modulat d carrier current for the line. For reception at the same terminal station the oscillator is disconnected from the light valvefand the inc/oming carrier picture currents are impressed upon this valve thereby causing it to pass to the film holder, which now holds the receiving film,- a varying beam of light 'corresponding to the light intensities of the elemental areas of the picture which is being transmitted from thev distant station, .The scanning at the terminals may be carried on in accordance ,with standard practice well known in the art. A suitable scanning system isv dis- 4closed in Patent No. 1,706,032, March 19, 1929, of M. B. Long. "The level. ,of the line carrier currents maybe raised at the trans-'- mitt terminal, to minimize the effect of distur nces entering by way vof the line, by

impressing some of the voltage of the oscillation generator upon the line in phase with the line currents, or the same result may be obtained optically by shunting some of the carrier light beam to the photoelectric cell.

A more detailed description of the inven-A tion follows and is illustrated in the accompanying drawing.

Figure 1 is a general schematic arrangement of an embodiment of this invention showing the terminal apparatus of two interconnected stations.

Fig. 2 is a detail, showin the li ht valve ribbon with relation to the ight va ve apertures when no current is flowing.

Fig. 3 is a curve, showing the variationv in the intensity of the illumination of the picture at the transmitter with time.

Fig. 4 is a curve, showing the photoelectric 'current resulting from lifrht modulation by In Fig. 1, in which two interconnected stai tions are shown, the terminal at the left, designated station lV, will be described as a transmitting station and the switch 10 at this station is accordingly thrown to the rightt' while the terminal at the right, designated station E, will be considered as a receiving station and the switch 10 of that station is also thrown to the right, thus connecting the terminal apparatus of the two stations for transmission and reception, res ivel 'At station W, light valve 2 0 is positioned in p v the path of light obtained from' any suitable source 2 1. A suitable lens system 22, 23- and 24 and apertured members 25 and 26 causenI beam of light to pass through the light Vv alvc and be focused on a small elemental area of the .picture where the light is modulated by the varying transparency ofthe elemental `areas of th picture. The picture 50 is shown as a film of varying transparency mounted on a transparent drum 51 which is given a light beam to successively scan the elemental vio .both t areas of the entire picture. It is preferable to mount the light valve and the picture holddrum so that the axis of the light valve ri l n and the drum are at right angles to each other. The light beam, after passing through the picture, is reected 90 by a prism or mirror 60 and passed throu h a ens system 61 to the light sensitive cell 0. The light sersitive cell, upon being activated by the varying light, causes varying electric. currents Ato be transmitted to the amplifier 80. The amplifier connects through the switch 81 to the cicuit 82 which in turn connects through blades 11 and 12 of switch to the outgoing. circuit 90. The outf go' circuit 90 connects through the filter 91 and the terminal amplifier 92 with the transmission line leading to -a receiving station. The l' ht beam activating the light sensitive cell 0 is a pulsating or carrier light beam and the variation in the intensity of the beam is caused by the light valve 20. The ribbon 27 of the li ht valve 20 is energized by au oscillator 30 w 'ch generates a sinusoidal wave of the order of 1,000 to 2,000 or more cycles per second. The oscillator 30 transmits its current through blades 13 and 14 of .switch 10`and a connecting circuit to the amplier where the oscillating current is amplified and the output current ofthe amplier passed through t e ribbon 27 of the -li ht valve thereby causing the llght valve rib n to oscillate at a substantia y constant frequency. The movement of the light valve ribbon thus causes the light beam to rapidly vary in intensity in accordance with the freuency of the output current of the oscillator. helbiasing battery 41 and the high resistance 42 aord means for adjusting the relation of the light valve ribbon with respect to the light valve aperture.

It has been found desirable in the transmission of pictures to provide for establishing a deimte ratio between the maximum and picture currents transmitted, and to provide for theV transmission of a definite minimum carrier current at all times even when no light reaches the light sensitive cell. The purpose of this is to avoid overloading the transmission circuit apparatus such as repeaters 'and also to avoid inducing cross-talk `in neighboring circuits., andv to insure that the .picture currents transmitted will be at an energy sucientto avoid serious interference from noise currents. 4Thismay be accomplished as shown in Fi 1 by using a part of the output current of t e oscillator and adding it to the amplified picturecurrents, lprovision being made for adjusting e phase and the magnitude of'the former current with'respect to the latter or vice versa. One way of adding a part of the oscillator output to the icture current is by means of the phase adjuster 85 and associated potentiometer 86 which `connects the output side of the amplifier 40 through the circuit comprising the blades 15 and 16 of the switch 10 and the switch 87 to the circuit 82. By means of this phase aduster, which may comprise a suitable networ of variable impedances arranged in accordance with principles well known to the art, and the associated potentiometer, the output current of the oscillator can be phased with the icture current `and the necessary unmoduated carrier current of the proper amplitude im ressed `upon the transmission circuit. With such ad'ustment the shunted carrier and the variab e picture currents add-direct ly. The variations' in light shades or tones of the picture are represented in this case by a current in the output circuit of the light sensitive cell 70, whose frequency is the uency of the current impressed upon the light valve ribbon and whose amplitude at any instant is proportional to the light transmitted by the picture. If the light valve is adjusted so that when there is no current flowing through its ribbon, the ribbon uncovers `one-half of the li ht `valve aperture, then the illumination wi have a sinusoidal form represented by a constant added to a sine function, and atthe-same frequency as the current impressed upon the light valve ribbon, but with a phase displacement between its sine form component and the car rier of va proximately 180 or multiple thereof. e width of the li ht valve ribbon should be equal to that of the i ht valve aperture and its maximum am litu shoulddie within one-ha f its width. It is obvious that the am lied photoelectric current will add direct y tothe shunted output current of the oscillator if the phase of the latter current is adjusted relatively to the former. The nhase adjuster 85 is adapted to 4bring about the necessary phase adjustment. 4This arrangement of the light valve in association with the light sensitive cell is such that the varying currents from the cell have the same. requency as the variations of the light transmitted by the light valve and also the'same frequency as the current from the oscillator operating the light valve. This arrangement is important for if the light valve ribbon were diEerently adjusted with respect to the light valve aperture with no current flowing through the ribbon, a double frequency in the light sensitive cell, for example, might be set up ,which would make it more diicult to add the desired carrierI to the picture current.

In adjusting the a paratus for transmitting a picturepit may advantageous to ascertain the amount of current transmitted by the brightest and by the darkest portions of e of vibration iis `50 ture arranged so that with nov current B;ribbon to the light va the picture and for determinin this a meas, uring device 83 is arranged so t at by throwing switch 81 to its upper closed posltion the output circuit of the amplifier 80- ma be connected to this measuring device. pon placing in the light beam various portions of the film 50, or any other object of varying transparency, its transparency at different 'spots may be measured and indicated by the measuri-n device 83 in such units as may be desired. or example, the measuring device may have a scale calibrated to read in per cent of the total light. In putting the apparatus in operation the operator having 15 ascertained the transparency of the lightest and the darkest portions of the picture may use this in adjusting the apparatus for transmission.

The same measuring device 83 which may comprise a suitable arranged vacuum tube amplifier -detector unit is also used for indicating when the phase difference of the photo-electric current and the shuntedoscillator output current is zero. In making such a comparison, both the switches 81and 87 are thrown into their upper closedA position, thereby causing the ltput from both the 'light sensitive cell 7 0 and the phase adjuster 85 to be impressed simultaneously upon the measuring device 83.. Also the ratio between the maximum and minimum picture currents, and the -magnitude of the shunted'oscillator output can be determined by the use of this measuring device 83 and pro er manipulation ofthe switches 81 and 87 orconnecting with the two circuits. The magnitude of the picture current can be adjusted in a number of ways well known to the art such as by. varying the voltage imy4o pressed upon the light sensitive cell.

The description so far has related primarily to the apparatus arranged for transmitting. When arranged for reception a part of the same apparatus as is employed for transmission is used and its connection is shown in the terminal station E at the right of Fig. 1. The switch 110 is thrown to the right and the relation of the-light valve ribbon 127 with respect to the light valve sperwing throu' h the ribbon the lightV valve is closed.. enti@ switch 1,10 l1s thrown to the right only blades 111 and 112 are in circuit thus connectin the circuit 190 with the i5 am lifier 140 and t esli ht valve ribbon 127 an the other four bla es 113, 114, 115 and 116 disconnect the oscillator 130 of the phase adjuster 185 from the .o ratingl circuit. The proper relationship o the light valve ve aperture may be established mechanically by adjusting the light valve jaws orelectrically by means of the biasing battery' 141 and the potentiometer 142. The incoming picturecurrent is am liflied bythe terminal amplifier 192, and en assed through the filter 191 and the amplier 140 to the light valve ribbon. The received picture current impressed on the 1i ht valve ribbon controls its light transmisslon in a manner to reproduce the light eiects of the distant picture and cause a light beam to be impressed upn the' photo-sensitive film `1500m thecylinder 151 which is moved in synchronisln and in phase with the picture at the transmitting station in accordance with standard practice well known in the art. Light from a steady source 121 is focused by the lens 122 on the light valve ribbon 127, and under control of the light valve is passed through the lenses 123 and 124 andthe aper- -tures 125 and 126 to the photo-sensitive lm 150.

It is to be noted as previously set forth that the same apparatus units so far as may be required are employed for reception as are used for transmission and the principal adjustment necessary to put the terminal equipment into proper relationship for transmission or for reception are the circuit ,changes governed by the switches 10and 110 at the terminals and the proper adjustments of the light valve ribbon with respect to the light valve aperture when no current is flowing. The interchangeability or dual use of the apparatus uni-ts for either transmission or reception simplifies the terminal equipment and its o eration.

The re ation of the light valve ribbon to the light valve aperture when no current is flowing is such that for transmission the light valve ishalf open and for reception the light valve is substantially closed as is shown at T and R, respectively, in Fig. 2. The primary reason for the vhalf open position when no current is flowing when transa varying illumination which when impressed on a light sensitive cell causes the latter to set upa varying photo-electric current of the samevfrequency as the current actuating the light valve. The closed position of the light valve when no current is flowing when receiving is due to the. fact that the photo-electric current approaches zero when a dark portion of the picture is-being trans- \mitted and amaximum vwhen the lightest portion of the picture is being transmitted. The adjustment of the relationship between the lightvalve ribbon and the light valve aperture as'described in connection with Fig. 1 is based upon passing the proper biasing `current through the .light valve ribbon to causel it to take. up the proper position for transmission or for reception and this vrecplires -adjusting thel biasing current when t e light valve 1s changed from transmitting to receiving or viceversa. In Fig. 2 is shown a mechanicalmeans for making this change. Two light valveapertures .T and R are used and permanently positioned in the apertured member 28 in proper relationfship to the light valve ribbon. Only one o these apertures is o n at a time and this may be mechanica y effected by any suitable aryrangement such as the opaque shutter 29 capable of being positioned in front of either aperture as shown in the drawing. However, even when the mechanical arran ment shown in Fig. 2 isused the electrical iasing control would probably be advantageous for fine adjustments.

The illumination of the pi'cture or film when plotted against time ma be represented by a curve as indicated iai ig. 3. The illumination output of a light va ve, properly adjusted, is approximatel a linear-function of the current input to t e light valve ribbon so that the distortion from this source is negligible. This pulsating illumination has been described herein as producing a carrier light beam and as already explained it is impressed by means of a suitable optical system upon elemental areas of the icture whose image is being transmitted. ne reason for emp oying a Ipulsating carrier light beamis to ma e possib e the use of an alternatin current amplifier and thus use a photo-e ectric current under conditions which permit of its being ampliiied with ordinary voice frequency alternatin current vaccum tube ampliers. The emp oymentof such apparatus permits its use on existing telephone or other transmission circuits. K

The pulsating carrier light beam in passing through the picture at the transmitter 'ismodulated in accordance with' the light density of the elemental areas o f the picture and t e current variationsresulting from the modulated light beam bein impressed u pon the photosensitive cell at t e transmitting station may be represented by a curve having a substantially uniform fundamental -freiency but of varying amplitude as shown in ig. 4. The fundamental frequency of the photo-electric current as heretofore explained is the'same asthat of the illumina-` "Ihe unmodulated carrier current which is impressed upon the transmission circuit at all times, irrespective of the photo-electric current, may lbe obtained as described in Fig. 1 by shuntin a part of the oscillator output current lto t e outgoing circuit. An alternative optical arrangement ispshown in Fig. 5. Il i the latter arrangement in place of the electrical shunt comprising the electrical phase adjusting apparatus, as shown in Fig. 1, a part of the carrier light beam is diverted by means of a partially trans arent mirror 210 positioned in the path of t e light beamv in such relation that it diverts a small percentage of the total light to the light sensitive cell 70. In the path of tbe llight diverted to the light sensitive cell, is a plane or preferably a concave mirror 211 and an adjustable aperture 212 arranged for directin and controlling the diverted light. By suc an optical arran ment the major portion of the carrier lig t beam passes through the mirror 210 to the picture and a minor portion of the light is diverted to the li ht sensitive cell. lVhen the apparatus is use for'receiving the semi-transparent mirror 210 is moved out of the path of the light beam into the osition' indicated at 213. The current pr uced in the light sensitive cell by the two parts of .to both figures.

What is claimed is:

1. In an electroticaltransmission system, means for prodJucing a light beam, an oscillatin current generator a light valve actuated y current from sai 4gexierator for vaiing the stren h of said beam at a peri icity equal to t at of said oscillating current, and means including a light sensitive element activated by said varying light beam for producing varying current of the same frequency as the current produced by said oscillator.

2. In an electro-optical transmission system, a transmission circuit, an. analyzing light beam, 'a light interrupter in the ath o said -light beam, means for contro ing the said light beam to'produce a modulated carrier current in said circuit, and a source of supplementary unmodulated carrier current for establishing a minimum transmission level in said circuit.

.3. In an electro-optical transmission s ysj tem, a ltransmission circuit, an analyzing li ht beam, a light interrupter in the path o? said light beam, a picture whose image is to be transmitted, means for controllin the said light beam to produce a modulate carrier current in said circuit varying according to the lights and shades of the said picture, and means for establishing a definite ratio between the maximum and minimum current representing the darkest and lightest areas of said ictu 4. n an electro-o tical transmission system, a light valve light beam, an oscillating current generator for actuating said light valve, a light sensitive element for producing photo-electric current of the same fre uency as the current produced by the said oscillator, and means for adding to the output of the said light sensitive cell current from thesaid oscillator of the same phase and frequency as that produced by the said light sensitive cell.

5. In an electro-optical transmission system, a transmission circuit, a light beam, a light interrupter rapidlyl and regularly interupting said beam thereby producing a i carrier light beam, a carrier current generavio tor actuating said interrupter, a light controlling means modulating the'said carrier light beam, a light sensitive element excited by said modulated carrier light beam fthereby producing modulated photo-electric current which is impressed upon the said transmission circuit, 'and a phase adjuster shunt for applying carrier current pulsations produced by the said carrier current generator to the said transmission circuit in phase with said photo-electric current.

6. In an electro-optical system for the transmission of pictures, a source of light, a picture whose image is transmitted, a light lvalve positioned between said light. source juster circuit shunting a part of the input pulsating current transmitted to said light valve to .the output'circuit of the said light sensitive cell thereby impressing on the output circuit unmodulated carrier current which 1s applied to the outgoing transmission line even when the picture current from the photo-'electric cell is zero, and means for ad- .)usting the amplitude of this additional carrier to control the level difference corresponding to the maximum contrast in the said picture.

7. In an electro-optical transipission systemntermmal equipment comprising a source ofl light, an electro-mechanical li ht v lve for controlling a beam of light rom said source, -transmitting means including an oscillator for actuating said light valve for for either transmission or for reception, a li ht valve having two apertures so related with the light valve ribbon that when no current is lowin through the said 'ribbon one aperture is su stantially half open andthe other aperture substantially closed, one of said apertures being adapted for transmitting. a carrier light eam for producing carrier current forl transmission and the second of said apertures being adapted for controlling the intensity of a source of light in accordance with received. current for reception.

9. The combination with optical means for producing and focusing a beam of light, of light controlling means for controlling a characteristic of said light, means for causing signals received from a distant point to control said light controlling means, means for holding a light-sensitive element in such a position that said beam is focused thereupon for obtaining a visible indication of the signals from said distant point, a light-'sensitive current-controlling element, and means for transmitting si als to a distant point comprising said lig t-sensitive current-controlling element controlled by light from said beam.

10. In an electro-optical transmission system, a transmission circuit, a signal controlled light beam, means for interrupting said beam to produce modulated carrier current insaid circuit, and means for producing an additional component of unmodulated carrier current in said circuit for establishing a minimum transmission level therein.

`11. In an electro-optical transmission system, a transmission circuit, a signal controlled light beam, means for interrupting said beam to roduce modulated carrier curuent in said circuit, and means for producing an additional component of unmodulated carrier current in said circuit in phase with saidmodulated carrier current..

In testimony whereof, I. have signed my name to this speciication this '17th day of February, 1927,

FREDERICK W. REYNOLDS.

producing carrier current modulated in ac' cordance with the tone values of the elemental areas of a picture to be transmitted, receiving means inc uding a circuit for impressing received p ictiire current u on said light value, and switching means or converting said equipment from' the transmitting to the receiving condition by disconnectingsaid osi cillator f rom saidV light valve and connecting the circuit ofathe receiving means thereto. y

8. f In an electro-optical transmission system, an optical arrangement adaptedvfor use lso 

